Tripodal Binding Units for Self-Assembled Monolayers on Gold: A Comparison of Thiol and Thioether Headgroups

Abstract

Whereas thiols and thioethers are frequently used as binding units of oligodentate precursor molecules to fabricate self-assembled monolayers (SAMs) on coinage metal and semiconductor surfaces, their use for tridentate bonding configuration is still questionable. Against this background, novel tridentate thiol ligands, PhSi(CH(2)SH)(3) (PTT) and p-Ph-C(6)H(4)Si(CH(2)SH)(3) (BPTT), were synthesized and used as tripodal adsorbate molecules for the fabrication of SAMs on Au(111). These SAMs were characterized by X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The PTT and BPTT films were compared with the analogous systems comprised of same tripodal ligands with thioether instead of thiol binding units (anchors). XPS and NEXAFS data suggest that the binding uniformity, packing density, and molecular alignment of the thiol-based ligands in the respective SAMs is superior to their thioether counterparts. In addition, the thiol-based films showed significantly lower levels of contamination. Significantly, the quality of the PTT SAMs on Au(111) was found to be even higher than that of the films formed from the respective monodentate counterpart, benzenethiol. The results obtained allow for making some general conclusions on the specific character of molecular self-assembly in the case of tridentate ligands.

Scheme 1

The structures of the tripod ligands addressed in this study (PTT, BPTT, PTET, and BPTET), along with the structure of the monodentate reference ligand BPT.

Scheme 2

Three-step synthesis of PTT and BPTT, starting from PhSiCl₃ and p-Ph-C₆H₄SiCl₃, respectively.

Figure 1

Molecular structures of the two crystallographically independent molecules in the crystal of BPTT (probability level of displacement ellipsoids 50%). Selected bond lengths (Å) and angles (deg): Si1–C1 1.8784(11), Si1–C2 1.8745(11), Si1–C3 1.8691(11), Si1–C4 1.8667(11), S1–C1 1.8179(12), S2–C2 1.8182(12), S3–C3 1.8182(11); C1–Si1–C2 104.85(5), C1–Si1–C3 110.68(5), C1–Si1–C4 112.66(5), C2–Si1–C3 109.27(5), C2–Si1–C4 110.73(5), C3–Si1–C4 108.59(5), S1–C1–Si1 115.30(6), S2–C2–Si1 113.60(6), S3–C3–Si1 112.67(6); Si21–C21 1.8762(13), Si21–C22 1.8764(12), Si21–C23 1.8797(12), Si21–C24 1.8691(11), S21–C21 1.8189(14), S22–C22 1.8181(12), S23–C23 1.8129(13); C21–Si21–C22 110.59(6), C21–Si21–C23 106.39(6), C21–Si21–C24 110.59(5), C22–Si21–C23 109.07(5), C22–Si21–C24 109.60(5), C23–Si21–C24 110.55(5), S21–C21–Si21 110.01(7), S22–C22–Si21 111.03(6), S23–C23–Si21 111.18(6).

Figure 2

Normalized XPS C 1s spectra (open circles) of the PTT, BPTT, PTET, BPTET, and BPT SAMs on Au(111). The respective fits (solid lines), including the spectrum decomposition in the case of PTET, and a background (dotted line) are also shown.

Figure 3

Normalized XPS S 2p spectra (open circles) of the PTT, BPTT, PTET, BPTET, and BPT SAMs on Au(111). The decomposition of these spectra into individual contributions (solid lines) and a background (dotted line) is also shown.

Figure 4

C K-edge NEXAFS spectra of the BPT, PTT, BPTT, PTET, and BPTET SAMs on Au(111) acquired at an X-ray incidence angle of 55°.